Chemical Reactions

Chemical ReactionsThe change of one or more substances into

other substances having different composition and properties is called a chemical reaction. E.g.--

C(s) + O2(g) CO2(g) 2H2(g + O2(g) 2H2O(g)In a chemical reaction, the substances

which react together are called reactants whereas the new substances formed are called products.

Reactants Product

Types Of Chemical Reactions

Combination ReactionsDecomposition ReactionsDisplacement ReactionsDouble - Displacement ReactionsOxidation and Reduction Reactions

B C A B C+ +

Combination Reaction

A

A reaction in which two or more substances (elements or compounds) combine together to form a new substance is called a combination reaction.

Many combustion reaction are also examples of combination reaction.

where A,B,C and ABC represent elements or compounds.

H2(g) + Cl2(g) 2HCl(l)Mg(s) + Cl2(g) MgCl2(s)Fe(s) + S(s) FeS(s)MgO(s) + H2O(l) Mg(OH)2(aq)

Some Examples Of Combinaton Reactions

Decomposition Reaction

A B C A C+ +B

A reaction in which a substance is broken down into two or more simpler substances is known as decomposition reaction.

A decomposition reaction is opposite of combination. A decomposition reaction takes place only when some energy in form of heat, light or electricity is supplied to the reactant.

where A,B,C and ABC represent an element or compound .

Some Examples Of Decomposition ReactionsZnCO3(g) ZnO(s) + CO2(g)2H2O (l) 2H2(g) + O2(g)CaCO3(s) CaO (s) + CO2 (g)2H2O2(l) 2H2O(l) + O2(g)

Types Of Decomosition Reactions

Thermal decomposition reaction Decomposition caused by heating

Electrolytic decomposition (electrolysis) reaction

Decomposition reaction caused by electricity

Photodecomposition reaction Decomposition reaction caused by

light

Displacement Reaction

Y Z+X X ZY +

A reaction in which one part (an atom or a group of atoms) of a molecule is replaced by another is called a displacement reaction.

where X,Y,Z represent an element or compound .

Some Examples Of Displacement Reactions

Zn(s) + 2HCl(dil) ZnCl2(aq) + H2(g) 2KBr(aq) + Cl2(aq) 2KCl(aq) +

Br2(aq) CuSO4(aq) + Zn(s) Cu(s) + ZnSO4(aq) Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

Z+X X ZY +W WYW

Double-Displacement Reaction

Z+X X ZY +W WY

A reaction in which the two reacting ionic compounds exchange their corresponding ions is called a double-displacement reaction.

where W,X,Y,Z represent an element or compound.

Z+X X ZY +W WY

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Ni(NO3)2(aq) + 2NaOH(aq) Ni(OH)2(s) +NaNO3(aq)

2KBr(aq) + BaI2 (aq) 2KI(aq)+BaBr(aq)

Pb(CH3COOH)2(aq)+2HCl(aq) PbCl2(s)+CH3COOH(aq)

Some Examples Of Double-Displacement Reactions

Oxidation-Reduction Reaction

Oxidation Reaction: Any process involving addition of oxygen, removal of hydrogen and/or loss of electron is known as oxidation reaction.

Example: Addition of oxygenP4(s) + 5O2(g) 2P2O5(s) (oxidation of P4) Removal of hydrogenH2S(aq) +Br2(aq) 2HBr(aq) + S(s) (oxidation of H2S) Loss of electronAl (s) Al3+(aq) + 3e- (oxidation of Al)

Oxidising Agent : The substance which brings about oxidation of other substances is called an oxidising agent.

Example: 1. KMnO4 (Potassium Permanganate) 2 .H2SO4 (Conc. Sulphuric acid)

Reduction Reaction: Any process involving removal of oxygen, addition of hydrogen and/or gain of electron is known as reduction reaction.

Example: Removal of oxygen Fe2O3(s) + 3CO(g) Fe(s) + 3CO2(g) (reduction of Fe2O3)

Addition of hydrogen H2S(aq) + Cl2(g) 2HCl(aq) + S(s) (reduction of Cl)

Gain of electron Cu2+(aq) + 2e- Cu(s) (reduction of Cu2+)

Reducing Agent: The substance which brings about reduction of other substance is called a reducing agent.

Example: 1. H2 (Hydrogen) 2.SO2 (Sulphur Dioxide)

“Reduction is the reverse of oxidation”

“Oxidation and reduction are mutually dependent, i.e. oxidation and reduction are reciprocal. Thus, in a reaction if a substance oxidises, another reduces.”